Calcium (1-100 micron M) caused a concentration and time-dependent increase in the phosphorylation of several synaptic cytosolic proteins. These effects of calcium were most apparent for proteins with molecular weights (Mr) of 50,000, 55,000 and 60,000. The administration of morphine, in vivo (3.0-30 mg/kg) caused a dose and time dependent increase in the phosphorylation of the 50,000, 55,000 and 60,000 molecular weight proteins compared to the values obtained from saline-treated controls. This effect of morphine was reversed by the specific narcotic antagonist, naloxone. Since calmodulin mediates calcium-dependent phosphorylation of synaptosomal cytosolic proteins, the effects of opiates on calcium-dependent protein phosphorylation may be through an action on this specific calcium binding protein.